An electrostatic MEMS switch is a switch operated by an electrostatic charge and manufactured using MEMS techniques. A MEMS switch can control electrical, mechanical, or optical signal flow. MEMS switches have typical application to telecommunications, such as DSL switch matrices and cell phones, Automated Testing Equipment (ATE), and other systems that require low cost switches or low-cost, high-density arrays.
As can be appreciated by persons skilled in the art, many types of MEMS switches and related devices can be fabricated by either bulk or surface micromachining techniques. Bulk micromachining generally involves sculpting one or more sides of a substrate to form desired three-dimensional structures and devices in the same substrate material. The substrate is composed of a material that is readily available in bulk form, and thus ordinarily is silicon or glass. Wet and/or dry etching techniques are employed in association with etch masks and etch stops to form the microstructures. Etching is typically performed on the frontside and backside of the substrate. The etching technique can generally be either isotropic or anisotropic in nature. Isotropic etching is insensitive to the crystal orientation of the planes of the material being etched (e.g., the etching of silicon by using a nitric acid as the etchant). Anisotropic etchants, such as potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), and ethylenediamine pyrochatechol (EDP), selectively attack different crystallographic orientations at different rates, and thus can be used to define relatively accurate sidewalls in the etch pits being created. Etch masks and etch stops are used to prevent predetermined regions of the substrate from being etched.
On the other hand, surface micromachining generally involves forming three-dimensional structures by depositing a number of different thin films on the top of a silicon wafer, but without sculpting the wafer itself. The films usually serve as either structural or sacrificial layers. Structural layers are frequently composed of polysilicon, silicon nitride, silicon dioxide, silicon carbide, or aluminum. Sacrificial layers are frequently composed of polysilicon, photoresist material, polyimide, metals or various kinds of oxides, such as PSG (phosphosilicate glass) and LTO (low-temperature oxide). Successive deposition, etching, and patterning procedures are carried out to arrive at the desired microstructure. In a typical surface micromachining process, a silicon substrate is coated with an isolation layer, and a sacrificial layer is deposited on the coated substrate. Windows are opened in the sacrificial layer, and a structural layer is then deposited and etched. The sacrificial layer is then selectively etched to form a free-standing, movable microstructure such as a beam or a cantilever out of the structural layer. The microstructure is ordinarily anchored to the silicon substrate, and can be designed to be movable in response to an input from an appropriate actuating mechanism.
Many current MEMS switch designs employ a cantilievered beam (or plate), or multiply-supported beam geometry for the switching structure. In the case of cantilevered beams, these MEMS switches include a movable, bimaterial beam comprising a structural layer of dielectric material and a layer of metal. Typically, the dielectric material is fixed at one end with respect to the substrate and provides structural support for the beam. The layer of metal is attached on the underside of the dielectric material and forms a movable electrode and a movable contact. The layer of metal can form part of the anchor. The movable beam is actuated in a direction toward the substrate by the application of a voltage difference across the electrode and another electrode attached to the surface of the substrate. The application of the voltage difference to the two electrodes creates an electrostatic field, which pulls the beam towards the substrate. The beam and substrate each have a contact which is separated by an air gap when no voltage is applied, wherein the switch is in the “open” position. When the voltage difference is applied, the beam is pulled to the substrate and the contacts make an electrical connection, wherein the switch is in the “closed” position.
One of the problems that faces current MEMS switches having a bimaterial beam is curling or other forms of static displacement or deformation of the beam. The static deformation can be caused by a stress mismatch or a stress gradient within the films. At some equilibrium temperature, the mismatch effects could be balanced to achieve a flat bimaterial structure, but this does not fix the temperature dependent effects. The mismatch could be balanced through specific processes (i.e., deposition rates, pressures, method, etc.), through material selection, and through geometrical parameters such as thickness. This bimaterial structure of metal and dielectric introduces a large variation in function over temperature, because the metal will typically have a higher thermal expansion rate than the dielectric. Because of the different states of static stress in the two materials, the switch can be deformed with a high degree of variability. Switch failure can result from deformation of the beam. Switch failure results when electrical contact is not established between the movable and stationary contacts due to static deformation or because of the deformation introduced as a function of temperature. A second mode of failure is observed when the movable contact and the stationary contact are prematurely closed, resulting in a “short”. Because of the deformation of the beam, the actuation voltage is increased or decreased depending on whether it is curved away from the substrate or towards the substrate, respectively. Because of this variability, the available voltage may not be adequate to achieve the desired contact force and, thus, contact resistance.
Typically, the beam of a MEMS switch is restored to an “open” position from a “closed” position by reducing the actuation voltage an amount sufficient for the resilient forces of the beam to deflect the beam back to the “open” position. The contacts of a MEMS switch frequently adhere to one another due metallurgical adhesion, cold welding, or hot welding forces. These forces are sometimes greater than the resilient forces of the beam, thus preventing the deflection of the beam to the “open” position. In such cases, switch failure results because the beam does not return to the “open” position. Therefore, it is desired to have a MEMS switch having a mechanism for generating a force to return the beam to an “open” position.